Printed wiring board fabrication generally employs a copper-clad laminate produced by laminating a copper foil on a surface of an insulating film such as a polyimide film by means of an adhesive.
To produce the copper-clad laminate, the copper foil is bonded to the insulating film coated with an adhesive layer by the application of heat and pressure. Accordingly, production of the copper-clad laminate inevitably involves the handling of single copper foil. The copper foil, however, becomes limper with reduction of thickness. For the copper foil to be handled singly, the lower limit of thickness is approximately 9 to 12 μm. The copper foil having any smaller thickness is extremely difficult to handle such that it must be fixed on a support. When a copper-clad laminate includes such an extremely thin copper foil that is bonded to an insulating film with an adhesive, fabrication of a wiring pattern produces a printed wiring board that is liable to suffer warpage due to thermal shrinkage of the adhesive that bonds the copper foil. In particular, there has been a need for printed wiring boards reduced in thickness and weight to meet the size and weight reduction of electronic equipment. It has been increasingly difficult to meet such printed wiring board needs with the above three-layer copper-clad laminate consisting of an insulating film, an adhesive and a copper foil.
Accordingly, the three-layer copper-clad laminate has been replaced with a two-layer laminate in which a metal layer is directly overlaid on an insulating film. This two-layer laminate is produced by depositing a seed-layer metal on a surface of the insulating film such as a polyimide film, by electroless plating, deposition or sputtering. The metal deposit is subsequently plated with copper and is coated with a photoresist, followed by photoexposure and development. And etching is performed to form a desired wiring pattern. In particular, because the metal (Cu) layer is thin, the two-layer laminate is suitable for production of very minute wiring patterns at wire pitches of less than 30 μm.
Patent Document JP-A-2003-188495 discloses a printed wiring board fabrication process comprising etching a metal-coated polyimide film to create a pattern, in which the metal-coated polyimide film includes a first metal layer provided on the polyimide resin film by a dry metal layer production process and a second conductive metal layer plated on the first metal layer, wherein the etching is followed by a rinsing treatment of the etched surface with an oxidant. Patent Document 1 discloses Example 5 in which a nickel-chrome alloy was deposited in a thickness of 10 nm by plasma deposition, and subsequently copper was deposited in a thickness of 8 μm by plating.
Although the two-layer metal-coated polyimide film fabricated as described above enables production of minute wiring patterns, it has the following problem. That is, when a metal layer is deposited on a surface of the polyimide film substrate by plasma deposition, partial chemical bonding often occurs in the surface of the polyimide film between the metal deposited and a component of the polyimide. Such metal is difficult to remove by etching. Consequently, the polyimide film on which wiring patterns are formed often contains a trace amount of the metal combined with the polyimide film. The metal remaining in the polyimide film surface can lower insulating properties between wires. Meanwhile, it is conventional practice to plate the wiring pattern before mounting an electronic component as illustrated in FIG. 7. The metal of a first metal layer 15 often bonds a component of a polyimide film 11, and the metal may remain in the surface of the polyimide film 11 surface. The bonding of the residual metal 30 is sometimes a physical bond on the polyimide film 11 and is sometimes a chemical bond between the metal and a polyimide film component. In the case of the chemical bonding, it is difficult to completely remove the residual metal 30 by etching.
After the wiring patterns of the first metal layer 15 and second metal layer 20 have been formed on the polyimide film 11, they are often coated with a plating layer 25 for protection, generally by electroless plating. The residual metal 30 in the polyimide film causes the plating metal to deposit therefrom in the electroless plating. For example, as indicated with the numeral 31 in FIG. 7, a metal 31 deposited on the residual metal 30 can deteriorate the inherent electrical insulating properties of the polyimide film. The metal 31 deposited on the surface of the polyimide film 11 has been found to cause deteriorated electrical insulating properties between wires because of migration between the wires via the metal deposit 31. Particularly, because of this migration, the wiring pattern immediately after manufactured exhibits good insulation resistance between wires but drastically lowers the insulation resistance after continuous voltage application in excess of 1000 hours.